Re: [Xen-devel] [PATCH v2] libx86: Work around GCC bug with ebx output constrants

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 19.11.18 at 15:45, <andrew.cooper3@xxxxxxxxxx> wrote:
> Versions of GCC before 5 can't compile cpuid.c, and fail with the rather 
> cryptic:
> 
>   In file included from lib/x86/cpuid.c:3:0:
>   lib/x86/cpuid.c: In function ‘x86_cpuid_policy_fill_native’:
>   include/xen/lib/x86/cpuid.h:25:5: error: inconsistent operand constraints 
> in an ‘asm’
>        asm ( "cpuid"
>        ^
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> 
> v2:
>  * GCC 5 is fine.  Its cpuid instrinct has none of the PIC workarounds thant 
> 4.9 have.
>  * Fix 64bit builds with larger models.

It is rather odd that 64-bit is also affected - the error gets raised
even when there's no use of %rbx for GOT accesses. By when
they need a callee-saved register, they indeed use %rbx instead
to the ABI-suggested %r15.

> --- a/xen/include/xen/lib/x86/cpuid.h
> +++ b/xen/include/xen/lib/x86/cpuid.h
> @@ -20,21 +20,50 @@ struct cpuid_leaf
>      uint32_t a, b, c, d;
>  };
>  
> +/*
> + * Versions of GCC before 5 are unable to cope with %rBX output constraints
> + * when compiling Position Independent Code, and produce a rather cryptic
> + * error:
> + *    error: inconsistent operand constraints in an ‘asm’
> + *
> + * To work around the issue, use a separate register to hold the the %rBX
> + * output, and xchg twice to leave %rBX preserved around the asm() statement.
> + */
> +#if defined(__PIC__) && __GNUC__ < 5 && !defined(__clang__) && 
> defined(__i386__)
> +# define XCHG_BX "xchg %%ebx, %[bx];"
> +# define BX_CON [bx] "=&r"
> +#elif defined(__PIC__) && __GNUC__ < 5 && !defined(__clang__) && \
> +    defined(__x86_64__) && (defined(__code_model_medium__) || \
> +                            defined(__code_model_large__))
> +# define XCHG_BX "xchg %%rbx, %q[bx];"
> +# define BX_CON [bx] "=&r"
> +#else
> +# define XCHG_BX ""
> +# define BX_CON "=&b"

The & is unnecessary here I think. Preferably with it dropped
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

Jan


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